This invention relates generally to phase change memory devices.
Phase change memory devices use phase change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, for electronic memory application. One type of memory element utilizes a phase change material that may be, in one application, electrically switched between a structural state of generally amorphous and generally crystalline local order or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states. The state of the phase change materials are also non-volatile in that, when set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance value, that value is retained until changed by another programming event, as that value represents a phase or physical state of the material (e.g., crystalline or amorphous). The state is unaffected by removing electrical power.
A transistor or a diode may be connected to the phase change material and may serve as a select device to access the phase change material during programming or read operations. The transistor or diode is typically formed in or on the top surface of a silicon single crystal substrate. Transistors may take up a relatively large portion of the memory chip, and therefore may increase the memory cell size, thereby adversely affecting the memory capacity and cost/bit of a memory chip.